The present invention relates to a vehicle ABS control for controlling a brake fluid pressure to prevent wheel locking on braking, a vehicle motion control for controlling the brake fluid pressure to prevent unwanted vehicle behavior due to an excessive over-steer condition or an excessive under-steer condition during steering operation, and a vehicle traction control for controlling the brake fluid pressure to optimize the driving forces of the drive wheels during starting operation.
An ABS brake fluid pressure control system in general compares a wheel speed with a pseudo vehicle body speed (approximating the vehicle speed) to determine a slip rate of each wheel. If the slip rate exceeds a predetermined level, the control system starts the ABS control, and controls the fluid pressure applied to the associated wheel cylinder to increase, hold and decrease the brake fluid pressure in a manner to prevent wheel locking.
A brake fluid pressure control system shown in a Japanese Patent Provisional Publication No. 9(1997)-95229 is arranged to prevent pulsation of a wheel cylinder pressure and reduce operating noises by performing, in an ABS control operation, a pulse width (or pulse duration) modulation control (PWM control) based on a duty ratio control to one or more solenoid valves in a brake actuator for varying the brake fluid pressure.
This system uses the PWM control in opening and closing a solenoid valve in the ABS control and varies the duty ratio to effectively reduce pulsation of the wheel cylinder pressure.
Moreover, in consideration of deviation, from a desired correspondence, of an actual correspondence between a PWM-controlled duty ratio and a coil current due to changes in the resistance of the coil or solenoid and the source voltage caused by variation in the temperature, the control system determines a PWM signal D1xe2x80x2 by the following equation.
D1xe2x80x2=D1xc3x97100xc3x97I0/Ixe2x80x20
In this equation, D1 is an original PWM signal before correction, I0 is a coil current at a duty ratio of 100% in a theoretical relationship between the duty ratio and the coil current, and Ixe2x80x20 is a coil current at the duty ratio of 100% in the coil current versus duty ratio characteristic curve, determined by the original PWM signal.
In the present invention, attention is directed to a relation between an operating condition, such as a coil temperature of a solenoid valve, and reduction of pulsation.
FIG. 17 shows control waveform according to a PWM control. The PWM control includes a T1 pressure increase interval and a T2 pressure increase interval. The T1 pressure increase interval is a short period immediately after the solenoid valve is turned OFF. During this, the PWM control is still inoperative. The T2 pressure increase interval is a period during which the PWM control is operative (the PWM duty ratio is effective). The T1 pressure increase time interval, the T2 pressure increase time interval and the duty ratio are preliminarily determined to achieve a pressure increase quantity equaling a moderate pressure increase quantity in the normal ABS control. If the coil temperature or the ambient temperature varies beyond limits A1 and A2 shown in FIG. 15, then the target current and the amount of the valve lift vary for the same duty ratio, so that the system becomes unable to achieve a desired moderate pressure increase, as shown at A in FIG. 17.
As shown in FIG. 15, a region of good PWM control performance is only part of a region of good normal ABS performance. The normal ABS control mode is a mode in which the PWM control based on the duty ratio control is not performed, and the controller outputs the drive signal simply made up of constant on signal and off signal. In the PWM control mode, by contrast, the drive signal is produced according to the PWM control. In the PWM mode, the control system controls the current supplied to the coil according to the PWM control, and this current is affected by variation in the temperature of the coil, so that the control system cannot achieve a desired moderate pressure increase. Therefore, the performance of the PWM control is satisfactory only in the narrow region shown in FIG. 15, and the performance becomes worse outside the region defined between A1 and A2. The control system may be able to achieve a desired moderate pressure increase by varying the PWM control appropriately when the temperature becomes closer to A1 or A2. This arrangement, however, requires costly microcomputer and complicated control program to continuously vary PWM.
It is therefore an object of the present invention to provide a brake fluid pressure control for reducing cost and preventing undesired pulsation.
According to the present invention, a brake fluid pressure control system comprises a brake actuator, a sensor and a control unit.
The brake actuator comprises at least one solenoid valve for regulating a brake fluid pressure for at least one wheel of a vehicle. The sensor senses a vehicle operating condition. The control unit is for controlling the brake fluid pressure by producing a drive signal to drive the solenoid valve of the actuator in accordance with the vehicle operating condition sensed by the sensor, and for producing, as the drive signal, a PWM control signal according to a pulse width modulation in the case of existence of a predetermined PWM control condition, and a normal control signal comprising an on signal and an off signal in the case of nonexistence of the predetermined PWM control condition.